(In Scheme, a semicolon marks the beginning of a comment that continues
until the end of the line. So the lines beginning ;; are
comments.)

Changing the value of an already existing variable is very similar,
except that define is replaced by the Scheme syntax set!,
like this:

(set! variable-namenew-value)

Remember that variables do not have fixed types, so new-value may
have a completely different type from whatever was previously stored in
the location named by variable-name. Both of the following
examples are therefore correct.

In these examples, value and new-value are literal numeric
or string values. In general, however, value and new-value
can be any Scheme expression. Even though we have not yet covered the
forms that Scheme expressions can take (see About Expressions), you
can probably guess what the following set! example does…

(set! x (+ x 1))

(Note: this is not a complete description of define and
set!, because we need to introduce some other aspects of Scheme
before the missing pieces can be filled in. If, however, you are
already familiar with the structure of Scheme, you may like to read
about those missing pieces immediately by jumping ahead to the following
references.

Lambda Alternatives, to read about an alternative form of the
define syntax that can be used when defining new procedures.

Procedures with Setters, to read about an alternative form of the
set! syntax that helps with changing a single value in the depths
of a compound data structure.)

See Internal Definitions, to read about using define other
than at top level in a Scheme program, including a discussion of when it
works to use define rather than set! to change the value
of an existing variable.